1. Field of the Invention
The present invention relates to a starter for cranking an internal combustion engine, and more particularly to a starter that includes a structure for preventing overheating.
2. Description of Related Art
It has been becoming a serious problem that the earth is being warmed up by carbon dioxide contained in the atmosphere. To cope with this problem by reducing fuel consumption in an automobile vehicle, great efforts are being made in downsizing automotive parts and components. In a starter motor for cranking an engine, its size and weight have been considerably reduced. On the other hand, the downsizing brings a problem of overheating. To overcome the overheating problem, various measures, such as improving heat-durability of materials used in the starter, have been taken. Such measures, however, have not been sufficiently effective to overcome the overheating problem.
If it is difficult to start an engine, or if a key-switch does not return to its original position, a large amount of current, such as several hundreds amperes, continues to flow through a starter for a long time. If this happens, the starter overheats and a further serious problem may follow. If a switch for supplying current to the starter is not opened due to its malfunction after the engine is successfully cranked up, several tens amperes may be continuously supplied to the starter. In this case, the starter continues to rotate at a high speed. Not only the starter is overheated but also commutator segments may be separated from a commutator surface by a high centrifugal force applied thereto. This may results in a complete loss of the starter.
Various proposals have been made as to ways and methods to shut off the current supply to the starter under such accidental situations mentioned above. For example, JP-A-10-66311 and WO-02/16763A1 propose to provide a fuse that melts away when the starter is overheated. Such a fuse may be provided in a main circuit for supplying current to the starter or in a pig tail connecting brushes. The fuse may be formed by reducing a cross-sectional area of a certain portion of the circuit. On the other hand, DE-10044081A1 and JP-A-59-185869 propose to form a solder-connection at certain position of a main circuit for supplying current to the starter, so that current supply is shut down by deformation or melting of the solder-connection when the starter is overheated.
The overheating problem, however, has not been sufficiently solved by those proposals. When the proposed fuse is used in the starter circuit, the fuse is blown away at a certain amount of current peculiar to that fuse. The amount of current flowing through the starter under no load condition is several tens amperes, while the amount of current is as high as several hundreds amperes when the starter is continuously operated without succeeding in cranking up the engine. Therefore, it is difficult to shut down the current in various levels with a single fuse. That is, if the fuse is designed to be blown at several hundreds of amperes, it is successfully blown by a high level of current, but it is not blown by a low current such as several tens of amperes.
On the other hand, if the fuse is designed to be blown by a low level current, there is a possibility that the current supply is unnecessary shut down. In addition, it is unavoidable that a resistance in the circuit is increased by such a fuse, resulting in decrease in the starter output. Because a large amount of current usually flows in the starter, the starter output is considerably reduced if there is an increase in resistance even in a small amount. To compensate such output decrease, the starter has to be made larger, which is contradictory to the downsizing.
In the technology of forming the solder-connection in the main circuit (proposed by DE-10044081A1 and JP-A-59-185869), it is expected that the solder-connection is deformed or disconnected without fail before components of the starter are damaged by overheating. For this purpose, the solder-connection has to be formed at a position where temperature is the highest and at a position that is closest to a power source such as a battery. Usually, the position where the temperature is the highest is a commutator surface which brushes slidably contact. However, it is difficult to form the solder-connection at a place where constant heat conduction from the commutator surface can be expected.
JP-A-59-185869 shows a solder-connection formed between a terminal bolt of a magnetic switch and a stationary contact. It further shows a resilient member for forcibly separating the circuit when the solder-connection melts away at a high temperature. DE-10044081A1 also proposes a similar resilient member for separating two contacts between which a solder-connection is formed. However, since the solder-connection is positioned, in both proposals, at a place far from a heat generating point, there is a possibility that an electric motor in the starter is damaged before the solder-connection shuts off the current supply. Further, since the resilient member is used in both proposals, a larger number of parts is required, resulting in a higher manufacturing cost. Further, DE-10044081 proposes to connect brushes and pig tails with solder. However, since the solder-connections are located far from the power source, it is difficult to shut off the current supply before other parts are damaged by heat.